It is generally known that a scanning optical system without distortion on a flat test surface (or a flat irradiation surface) is achieved by designing the scanning optical system like an optical system having telecentricity on the test surface side (i.e., image-side).
For example, an art related to a two-dimensional scanning optical system is disclosed in Patent Literature 1. In this art, the two-dimensional scanning optical system includes a cylindrical lens having a power only in a cross section along a main-scanning direction and a cylindrical lens having a power only in a cross section along a sub-scanning direction. The two cylindrical lenses are used so that telecentricity can be achieved in cross sections of the image-side space along the main-scanning direction and the sub-scanning direction.
In particular, as shown in FIG. 30, light emitting from a light source 41 passes through a collimator lens 42 and travels to a polygon mirror 43. The light traveled to the polygon mirror 43 reflects off the polygon mirror 43 so as to scan in the sub-scanning direction. The light reflected off the polygon mirror 43 so as to scan in the sub-scanning direction travels through a cylindrical lens 44 having a power only in a cross section along the sub-scanning direction, and travels to a polygon mirror 45. The light traveled to the polygon mirror 45 reflects off the polygon mirror 45 so as to scan in the main-scanning direction. The light reflected off the polygon mirror 45 so as to scan in the main-scanning direction travels through a cylindrical lens 46 having a power only in a cross section along the main-scanning direction, and travels to a scanning surface 47.
In this configuration, the cylindrical lens is placed so that the center of the polygon mirror can be located at a focal point on the object side of the cylindrical lens.
With this configuration, telecentricity is achieved in cross sections of the image-side space along the main-scanning direction and the sub-scanning direction.
In other words, the two-dimensional scanning optical system has a combination of the polygon mirror and the cylindrical lens, which has the power only in the cross section along the scanning direction of the polygon mirror, with respect to the main-scanning direction and the sub-scanning direction. Moreover, the cylindrical lenses and polygon mirrors are arranged in series. Thus, the two-dimensional scanning optical system can two-dimensionally scan and have telecentricity on the scanning 47 side.